There is a need in the art for comfortable, convenient and inexpensive means for prewarming air for breathing. For example, in the case of persons suffering from cardiac insufficiency, whether congenital or acquired as a result of heart damage resulting from injury or disease, the patient is commonly advised to engage in moderate exercise, as tolerated. One of the best of such exercises is walking, which is of course normally an outdoor activity. This, however, may lead to problems, particularly in cold weather. Under cold conditions, the heart is doubly stressed--partially by the physical demand on the muscles, requiring increased circulation to carry away lactic acid and other waste products of muscular activity, and partially by the additional increased circulation required to maintain the body temperature at a proper level. The latter stress does not signal its presence by muscular discomfort, as does the former, and the result is that the patient is unconsciously placing greater demands on his heart than he should. The same effect is commonly experienced by persons having no previous history of cardiac problems, as one is reminded every year by an upswing in incidence of heart attacks which can be directly associated with heavy exercise such as snow shovelling during periods of cold weather. One of the most direct mechanisms whereby the body is chilled is by inhalation of cold air which not only chills the tissues forming the walls of the components of the respiratory tract, but also chills the bloodstream itself by heat transfer which takes place in the alveole concurrently with the oxygenation of oxygen-depleted blood from the pulmonary artery. Thus, the demands placed on the heart for maintenance of body temperature can be alleviated and the above-mentioned problems can be overcome to an appreciable degree, by preheating air before it is inspired into the body.
Previous attempts to solve this problem have generally taken the form of interposing a porous barrier, for example of knit wool or spongy polymer, between the nose and/or mouth of the user and the external atmosphere. While successful to a degree, such measures suffer from certain disadvantages. For example, the interposed porous material necessarily involves some resistance to free air flow, and to that extent makes breathing more difficult. Also, there is an annoying tendency for mositure in exhaled breath to condense on contact with the cold ambient atmosphere, and to deposit as moisture or as ice crystals on the surface and in the pores of the porous material, creating a most uncomfortable condition. Perhaps more importantly, the mechanism whereby the warming of incoming air is accomplished is by direct heat exchange with warm exhaled air in the porous material. This necessitates that a certain amount of the exhaled air be trapped in the porous material, mixed with fresh incoming air to accomplish the desired heat exchange, and then reinhaled as part of the mixture. This, in turn, defeats pro tanto the advantage of the heat exchange, inasmuch as the mixture being breathed has already been partially depleted of oxygen, so that additional quantities of the mixed gases must be inhaled in order to maintain the required oxygen supply.